BACKGROUND
[0001] This disclosure generally relates to methods and systems for measuring a quantity
of liquid fuel in a fuel tank, such as a storage tank or other container. More particularly,
this disclosure relates to methods and systems for measuring the quantity of liquid
fuel in a fuel tank on an aircraft in a manner that does not require the presence
of electrical components in the fuel tank.
[0002] A need to continuously measure the quantity of liquid fuel in a fuel tank exists
in many commercial and military aircraft applications. For example, liquid-level sensors
are commonly used in the fuel tanks of aircraft. Liquid-level sensors are also used
to monitor liquid levels within storage tanks used for fuel dispensing.
[0003] Many transducers for measuring liquid level employ electricity. The electrical output
of such transducers changes in response to a change in the liquid level being measured,
and is typically in the form of a change in resistance, capacitance, current flow,
magnetic field, frequency, and so on. These types of transducers may include variable
capacitors or resistors, optical components, Hall Effect sensors, strain gauges, ultrasonic
devices, and so on.
[0004] Currently most fuel sensors on aircraft use electricity. For example, existing electrical
capacitance sensors require electrical wiring inside the tank, which in turn requires
complex installations and protection measures to preclude a safety issue under certain
electrical fault conditions. This electrical wiring requires careful shielding, bonding,
and grounding to minimize stray capacitance and further requires periodic maintenance
to ensure electrical contact integrity.
[0005] In the cases of commercial and military aviation, it is important for the flight
crew to know there is adequate fuel upload for a mission prior to each flight. It
is equally important for the crew to know during the flight that there is adequate
fuel remaining in the tanks to complete each flight safely. A simple and accurate
fuel quantity gauging system is needed. For a typical long-range transport aircraft,
it takes a quarter to a half pound of fuel to transport a pound of weight. Extra fuel
is dead weight and it takes fuel to transport that extra weight.
[0006] It would be advantageous if the amount of liquid fuel in a fuel tank could be measured
without introducing electrical current into the fuel tank and without using optical
techniques. An apparatus and method that provides the determination of the remaining
quantity of fuel in fuel tanks based on the ullage pressure and temperature changes
is disclosed in
US 4 553 431 A.
SUMMARY
[0007] The subject matter disclosed in detail below and defined in claims 9 and 1, is directed
to methods and systems for fuel quantity gauging that measure the quantity of liquid
fuel in a fuel tank on an aircraft directly without the need to accurately locate
fuel heights throughout the fuel tank using multiple fuel gauging probes. The method
may comprise the following steps performed while fuel is flowing out of the fuel tank:
(a) changing a volume of gas in the fuel tank (e.g., by injecting or venting gas)
during a time interval; (b) measuring a rate of change of the volume of gas in the
fuel tank during the time interval; (c) measuring a rate of flow of fuel out of the
fuel tank during the time interval; (d) measuring a first pressure and a first temperature
of the gas in the fuel tank at the start of the time interval; (e) measuring a second
pressure and a second temperature of the gas in the fuel tank at the end of the time
interval; and (f) calculating a quantity of fuel in the fuel tank based on the measurement
data acquired in steps (c) through (f). Step (f) is performed by a processing unit.
The calculation is simple and does not require the calculation of instantaneous fuel
volume topography. The computing power requirement is minimal. Unlike electrical and
electronic probes, no electric current in the fuel tank is required.
To meet aviation requirements, two completely independent sets of fuel gauges are
required. A typical aircraft may have a multiple-point electronic system for flight
and a magnetic or mechanical system available to the ground crew during fuel upload.
The system proposed herein could be used as the primary or secondary system. It may
also be used in conjunction with the current electronic system, making two independent
in-flight capable systems available to the crew. In an example, which does not form
part of the present invention, the methodology disclosed herein is not limited to
aircraft application but rather may also be used in land and marine vehicles as well
as stationary liquid fuel tanks.
[0008] One aspect of the subject matter disclosed in detail below is a system for measuring
a quantity of liquid fuel in a fuel tank on an aircraft, comprising: a first meter
that measures a rate of flow of gas through a gas line that is in fluid communication
with the fuel tank; a second meter that measures a rate of flow of fuel out of the
fuel tank via a fuel line; a first gauge that measures an ullage temperature in an
ullage of the fuel tank; a second gauge that measures an ullage pressure in an ullage
of the fuel tank; and a processing unit programmed to calculate a quantity of fuel
in the fuel tank based on measurement data from the first and second meters and from
the first and second gauges. The system may further comprise a fuel gauge connected
to receive and display symbology representing the quantity of fuel.
[0009] In accordance with some embodiments, the processing unit is programmed to calculate
a quantity of fuel in the fuel tank at a second time subsequent to a first time based
in part on respective ullage temperature and pressure measurements taken by the first
and second gauges at first and second times.
[0010] In accordance with other embodiments, the processing unit is programmed to: calculate
a change in mass of gas in the fuel tank by integrating an output of the first meter
over a time interval from a first time to a second time; calculate a change in volume
of fuel in the fuel tank by integrating an output of the second meter over the time
interval; and calculate a quantity of fuel in the fuel tank at the second time based
on the calculated changes in mass of gas and fuel in the fuel tank during the time
interval and respective ullage temperature and pressure measurements taken by the
first and second gauges at the first and second times.
[0011] Another aspect of the subject matter disclosed in detail below is a method for measuring
a quantity of liquid fuel in a fuel tank while fuel is flowing out of the fuel tank,
comprising: (a) changing a volume of gas in the fuel tank during a time interval that
starts at a first time and ends at a second time; (b) measuring a rate of change of
the volume of gas in the fuel tank during the time interval; (c) measuring a rate
of flow of fuel out of the fuel tank during the time interval; (d) measuring a first
pressure of gas in the fuel tank at the first time; (e) measuring a first temperature
of gas in the fuel tank at the first time; (f) measuring a second pressure of gas
in the fuel tank at the second time; (g) measuring a second temperature of gas in
the fuel tank at the second time; and (h) calculating a quantity of fuel in the fuel
tank based on measurement data acquired in steps (b) through (g), wherein step (h)
is performed by a processing unit. The method may further comprise closing a vent
in fluid communication with the ullage prior to step (a), wherein step (a) comprises
injecting gas into the fuel tank via a gas line during the time interval while the
vent is closed, and step (b) comprises measuring a rate of flow of gas into the fuel
tank via the gas line. In the alternative, the method may further comprise opening
a vent in fluid communication with the ullage prior to step (a), wherein step (a)
comprises venting gas out of the fuel tank via the open vent during the time interval,
and step (b) comprises measuring a rate of flow of gas out of the fuel tank via the
open vent. The method may further comprise displaying symbology representing the quantity
of fuel. In one implementation, steps (a) through (h) are performed onboard an aircraft.
[0012] A further aspect is a method for measuring a quantity of liquid fuel in a fuel tank
onboard an aircraft during flight, comprising: (a) changing a volume of gas in the
fuel tank during a time interval that starts at a first time and ends at a second
time; (b) measuring a rate of change of the volume of gas in the fuel tank during
the time interval; (c) measuring a rate of flow of fuel out of the fuel tank during
the time interval; (d) measuring a first pressure and a first temperature of gas in
the fuel tank at the first time; (e) measuring a second pressure and second temperature
of gas in the fuel tank at the second time; (f) calculating a change in mass of gas
in the fuel tank during the time interval; (g) calculating a change in volume of fuel
in the fuel tank during the time interval; (h) calculating the quantity of fuel in
the fuel tank at the second time based on the calculated changes in mass of gas and
volume of fuel in the fuel tank during the time interval, the first and second temperatures,
and the first and second pressures; and (i) displaying symbology representing the
quantity of fuel, wherein steps (f) through (h) are performed by a processing unit.
[0013] Other aspects of methods and systems for measuring the quantity of fuel in a fuel
tank are disclosed below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014]
FIG. 1 is a block diagram showing major components of a system onboard an aircraft
for converting liquid fuel into power.
FIG. 2 is a block diagram showing a fuel tank containing liquid fuel and ullage gas
and gauges for measuring pressure and temperature of the ullage gas at the start of
gas injection (indicated by the dashed arrow labeled "Gas ΔM").
FIG. 3 is a block diagram showing the same components as are depicted in FIG. 2 after
the injection of an amount of gas equal to ΔM (indicated by the solid arrow labeled
"Gas ΔM").
FIG. 4 is a flowchart showing steps of a method for direct measurement of the quantity
of liquid fuel in a fuel tank while gas is being injected into the fuel tank in accordance
with one embodiment.
FIG. 5 is a flowchart showing steps of a method for direct measurement of the quantity
of liquid fuel in a fuel tank while gas is being vented out of the fuel tank in accordance
with another embodiment.
FIG. 6 is a block diagram identifying components of a system for measuring a level
of liquid fuel in a fuel tank in accordance with some embodiments.
[0015] Reference will hereinafter be made to the drawings in which similar elements in different
drawings bear the same reference numerals.
DETAILED DESCRIPTION
[0016] Various embodiments of methods and systems for measurement of the quantity (i.e.,
volume or mass) of liquid fuel in a fuel tank on an aircraft will now be described
in detail for the purpose of illustration. It should be appreciated, however, that
in an example, which does not form part of the present invention, the methodology
disclosed herein is not limited to aircraft applications but rather may also be used
in land and marine vehicles as well as stationary liquid fuel tanks. At least some
of the details disclosed below relate to optional features or aspects, which in some
applications may be omitted without departing from the scope of the claims appended
hereto.
[0017] Fuel tanks in vehicles carry fuel that is used to operate the engines of a vehicle.
The fuel is flammable in the presence of oxygen or air. When fuel is used, the level
of fuel decreases. This decrease in the level of fuel results in a space filled with
gas increasing in size above the level of the liquid fuel in the fuel tank. The space
above the liquid fuel may contain air and fuel vapors. This space is referred to as
an "ullage".
[0018] Increased safety for fuel tanks may be provided through the use of an inert gas system.
The inert gas system may generate and distribute an inert gas to reduce the oxygen
content that may be present in the fuel tanks. In particular, the space (i.e., ullage)
above the surface of the fuel in the fuel tank is filled with an inert gas. The inert
gas displaces air that contains oxygen in the fuel tank. The inert gas may also displace
fuel vapors and other elements. This process is called "inerting". The inert gas reduces
the oxygen content in this space in a manner that reduces a possibility of a combustion
event, including ignition, detonation, or deflagration. The combustion event may be
the combustion of the fuel, fuel vapor, or both.
[0019] An on-board inert gas generation system (OBIGGS) may be used to generate oxygen-depleted
(i.e., inert) gas to inert the ullage in fuel tanks. Inerting the ullage portion of
the fuel tank reduces the oxidizing agents in the fuel tank and therefore reduces
the flammability of the vapor therein. This inert gas may be, for example, nitrogen,
nitrogen-enriched air, carbon dioxide, and other types of inert gases.
[0020] FIG. 1 is a block diagram showing major components of a system onboard an aircraft
for converting liquid fuel into power and inerting the fuel tank. The system comprises
an on-board inert gas generation system (OBIGGS) 2. Air is delivered to the OBIGGS
2. The OBIGGS 2 is in fluid communication with a fuel tank 4. Oxygen is separated
from air in the OBIGGS 2 and the remaining oxygen-depleted air is sent to the fuel
tank 4. The interior volume of the fuel tank 4 contains ullage gas 8 overlying liquid
fuel 10. The typical OBIGGS 2 separates oxygen and nitrogen, the two main components
of air. The oxygen is not used and nitrogen-enriched air is pumped into the fuel tank
4 to reduce the oxygen concentration of the ullage gas 8. In some embodiments, the
OBIGGS 2 produces nitrogen-enriched air from engine bleed air (i.e., pressurized air
that is bled from different engine compressor stages for pneumatic system consumers
on the aircraft). The ullage gas 8 can be vented overboard from the fuel tank 4. Fuel
from the fuel tank 4 is delivered to the engines or to an auxiliary power unit (APU)
6, thereby enabling the generation of power as the fuel is combusted. The OBIGGS 2
is normally turned on after fuel upload prior to each flight. For military aircraft,
the system is typically on. On commercial flights, the system may be turned off in
flight.
[0021] Gas (including ullage gas in a fuel tank) is compressible, which means that when
a given mass of gas is pressurized, its volume decreases, i.e., more mass can be compressed
into the same volume under increasing pressure. Both air and nitrogen-enriched air
are compressible and are ideal gases. Ideal gas behaviors are governed by the ideal
gas law:
where P is pressure, V is volume, M is amount (e.g., mass), R is the ideal gas constant,
and T is temperature. In SI units, P is measured in pascals, V is measured in cubic
meters, M is measured in moles, and T is measured in degrees Kelvin. R has the value
8.314 J·K
-1·mol
-1 or 0.08206 L·atm·mol
-1-K
-1 if using pressure in standard atmospheres (atm) instead of pascals, and volume in
liters instead of cubic meters.
[0022] In contrast to the compressibility of gas, liquid is incompressible, which means
that when a liquid is pressurized, its volume does not change. Liquid fuel is incompressible.
[0023] In various embodiments, the aircraft includes two major types of fuel metering systems:
an engine fuel flow meter and a fuel tank fuel quantity gauge. The engine fuel flow
meter is on the fuel line in the engine. It gives a very accurate instantaneous fuel
flow reading of fuel supplied to each engine. Summing the readings from all the fuel
flow meters and integrating over flight time gives the crew total fuel consumed in
flight. The problem with this system alone is that only the quantity of fuel going
into the engine is known. The fuel flow meters do not provide how much fuel is in
each fuel tank.
[0024] There are at least five types of fuel tank fuel quantity gauges used in aviation:
(1) sight glass, (2) mechanical, (3) electrical; (4) electronic, and (5) optical.
Because of fuel tank complexity, flight condition variation, and the attitude of the
aircraft is not constant, multiple location readings in the tank are needed for accuracy.
A typical small commercial aircraft may require a minimum of twenty fuel gauging probes
and a large aircraft may have over sixty. Total system weight is high and failure
of any one probe compromises accuracy. To convert the instantaneous probe data and
flight data to fuel quantity, a sophisticated algorithm is required. The algorithm
requires much computing power. When there is a design change such as plumbing re-routing
in the fuel tank, fuel quantity software update is required. In addition to fuel gauging
probes, the aircraft may require a probe compensator to compensate for variation in
fuel permittivity, a densitometer to measure fuel density, and a temperature sensor
to measure fuel temperature. In flights where most time is spent in level cruise,
the fuel and ullage interface line is relatively calm and level. Under these conditions,
level sensing gauges provide accurate data. In flights where an aircraft is maneuvered
constantly, these gauges may not provide desirable results.
[0025] The fuel gauging system proposed herein overcomes many of the shortcomings described
in the preceding paragraphs. For each fuel tank, the fuel gauging system comprises
an incoming gas line, a vent line with a valve, an ullage gas pressure gauge, and
an ullage gas temperature gauge. A pressure measurement and a temperature measurement
are taken during each fuel quantity reading.
[0026] FIG. 2 is a block diagram showing a fuel tank 4 containing liquid fuel 10 and ullage
gas 8, a pressure gauge 12 for measuring the pressure of the ullage gas inside the
fuel tank 4, and a temperature gauge 14 for measuring the temperature of the ullage
gas inside the fuel tank 4. The system shown in FIG. 2 also comprises one or more
injection nozzles (not shown) for injecting nitrogen-enriched air (indicated by the
dashed arrow labeled "Gas ΔM") into the fuel tank 4 via an incoming gas line (not
shown), a vent line 16 for fuel tank venting (pressure equalization) and discharging
ullage gas, and a climb/dive valve 18 on the vent line 16 to control venting. With
the valve 18 on the vent line 16 closed, a small amount (i.e., mass) of gas, indicated
by ΔM in FIG. 2, is injected into the fuel tank 4. The amount ΔM is limited by the
fuel tank overpressure design limit. The readings on the pressure and temperature
gauges 12, 14 seen in FIG. 2 are intended to indicate the ullage pressure and temperature
at the start (also referred to below as "Time 1") of a time interval (which time interval
ends at Time 2, where Time 2 - Time 1 = ΔT). The dashed arrow labeled "Gas ΔM" in
FIG. 2 indicates gas injection occurring at Time 1. The gas injection continues until
at least Time 2.
[0027] FIG. 3 is a block diagram showing the same components as depicted in FIG. 2 after
the injection of an amount of gas equal to ΔM (indicated by the solid arrow labeled
"Gas ΔM") during the time interval from Time 1 to Time 2. In other words, ΔM represents
the change in mass of the ullage gas during ΔT. As will be described in more detail
later, ΔM can be computed by integrating the rate at which gas is flowing into the
fuel tank 4 over the time interval from Time 1 to Time 2.
[0028] Although not indicated in FIGS. 2 and 3, liquid fuel 10 is also flowing out of the
fuel tank 4 as gas is being injected into the fuel tank 4. As will be described in
more detail later, a decrease in fuel volume ΔV can be computed by integrating the
rate at which fuel is flowing out of the fuel tank 4 over the time interval from Time
1 to Time 2.
[0029] As gas is injected into and fuel 10 flows out of the fuel tank 4, the pressure and
temperature of the ullage gas 8 changes, as indicated by the readings on pressure
and temperatures gauges 12, 14 seen in FIG. 3 as compared to the respective readings
on the same gauges seen in FIG. 2.
[0030] In accordance with one methodology, a first pressure measurement and a first temperature
measurement are taken at Time 1, as indicated by the readings of pressure gauge 12
and temperature gauge 14 shown in FIG. 2. A second pressure measurement and a second
temperature measurement are taken at Time 2, as indicated by the readings of pressure
gauge 12 and temperature gauge 14 shown in FIG. 3. As gas is added to the ullage 8
during the time interval from Time 1 to Time 2 (while the valve 18 of the vent line
16 is closed), since gas is compressible and the ullage is constrained, the measured
ullage pressure and ullage temperature should both increase. Conversely, the fuel
is incompressible. Therefore, although the ullage pressure increases as gas is injected,
the fuel volume does not change due to this increase in ullage pressure (although
it does change due to the flow of fuel to the engine).
[0031] In cases where OBIGGS is not available or for aircraft without an OBIGGS, engine
bleed air can be used as the injected gas. Engine bleed air can be hot, so pre-cooling
may be required. In the case of gas injection, the gas flow rate can be measured by
a gas flow meter located along the incoming gas line. By integrating gas flow rate
between Time 1 and Time 2, the gas mass change ΔM is obtained. The fuel flow rate
(gpm or Ib/hr) to the engine is measured at the engine and is a known quantity during
flight. By integrating fuel flow rate between Time 1 and Time 2, the fuel volume change
ΔV is obtained. This change in fuel volume will be equal and opposite to the change
in ullage volume during the time interval since the volume of the fuel tank is constant.
[0032] Another source of pressurized air comes from cabin air. For commercial aircraft,
the air in the cabin is changed constantly. The air is dumped overboard. For long-range
aircraft, this air may be used to pressurize the ullage. The pressure in an aircraft
cabin is typically set at 8,000 ft pressure altitude or eleven pounds per square inch.
The ambient air pressure at a cruise altitude of 35,000 ft is 3.5 pounds per square
inch. There is ample air pressure in the cabin air to run the fuel gauging system.
In addition, the cabin air is a wasted air; therefore it does not cost fuel for pressurization.
[0033] The ideal gas law is then used to calculate the ullage volume (V + ΔV) at Time 2.
The two gas laws at Times 1 and 2 are as follows:
The pressure P
1 and temperature T
1 are measured at Time 1 (at or after the start of gas injection); the pressure P
2 and temperature T
2 are measured at Time 2 (while gas injection continues); and ΔM is measured over the
time interval (from Time 1 to Time 2) during which gas is being injected. R is the
ideal gas constant. The initial ullage volume V and initial gas mass M in the fuel
tank 4 at Time 1 are the only two unknowns. The respective equations for thermodynamic
states at Times 1 and 2 are solved simultaneously for ullage volume V at Time 1 and
ullage gas mass M at Time 1. The ullage volume calculated in each instance is the
true ullage volume regardless of the shape of the ullage or how many gas bubbles make
up the ullage volume.
[0034] The difference between the interior volume of the fuel tank and the ullage volume
(V + ΔV) at Time 2 is the fuel volume at Time 2. Similarly, the fuel volume at a Time
3 (subsequent to Time 2) can be computed by using either the ideal gas laws for Times
2 and 3 or for Times 1 and 3. In each instance, the product of the fuel volume and
the fuel density is the fuel weight.
[0035] The measurement and calculation processes continue until the ullage gas pressure
reaches a pre-specified limit. The limiting ullage pressure is always below the fuel
tank overpressure design limit. When the limiting ullage pressure is reached, the
valve 18 is opened to allow the ullage to depressurize. During depressurization, the
ullage gas pressure and temperature measurements continue. The only difference is
that the change in the mass of ullage gas ΔM is now negative.
[0036] FIG. 4 is a flowchart showing steps of a method for direct measurement of the quantity
of liquid fuel in a fuel tank while gas is being injected into and fuel is flowing
out of the fuel tank in accordance with one embodiment. First, the vent valve is closed
(step 20). Then gas is injected into the fuel tank via a gas line during a time interval
that starts at time T and ends at Time (T + ΔT) while the vent valve is closed (step
22). The rates at which gas is flowing into and fuel is flowing out of the fuel tank
are measured throughout the time interval (step 24). In addition, the pressure and
temperature of the gas in the fuel tank are measured at time T (step 26) and at time
Time (T + ΔT) (step 28). A change in volume of fuel in the fuel tank during the time
interval ΔT is then calculated (step 30). Likewise a change in mass of gas injected
into the fuel tank during the time interval ΔT is calculated (step 32). The ullage
gas mass M and ullage gas volume V at Time T are then calculated using the respective
gas law equations for Times T and (T + ΔT) as previously described (step 34). The
fuel volume at Time (T + ΔT) can then be calculated (step 36). At the same time, a
determination is made whether the ullage gas pressure has reached the preset maximum
pressure or not (step 38). If the preset maximum pressure has been reached, then gas
injection is stopped (step 40). If the preset maximum pressure has not been reached,
then gas injection continues and the process returns to step 28, i.e., the pressure
and temperature of the gas in the fuel tank are measured again after the passage of
a second time interval that starts at Time (T + ΔT) and ends at (T + 2ΔT). Successive
datapoints can be acquired at regular time increments ΔT, i.e., at successive times
(T + nΔT), where n = 1, 2, 3, etc.
[0037] FIG. 5 is a flowchart showing steps of a method for direct measurement of the quantity
of liquid fuel in a fuel tank while gas is being vented and fuel is flowing out of
the fuel tank in accordance with another embodiment. First, the vent valve is opened
(step 60). Then ullage gas is vented out of the fuel tank via a vent line (e.g., using
a gas pump) during a time interval that starts at time T and ends at Time (T + ΔT)
while the vent valve is open (step 62). The rates at which gas and fuel are flowing
out of the fuel tank are measured throughout the time interval (step 24). In addition,
the pressure and temperature of the gas in the fuel tank are measured at time T (step
26) and at time Time (T + ΔT) (step 28). A change in volume of fuel in the fuel tank
during the time interval is then calculated (step 30). Likewise a change in mass of
gas being vented out of the fuel tank during the time interval is calculated (step
64). The ullage gas mass M and ullage gas volume V at Time T are then calculated using
the respective gas law equations for Times T and (T + ΔT) as previously described
(step 34). The fuel volume at Time (T + ΔT) can then be calculated (step 36). At the
same time, a determination is made whether the ullage gas pressure has reached zero
(relative to ambient pressure) or not (step 66). If the ullage gas pressure has reached
zero, the vent valve is closed (step 20). If the ullage gas pressure has not reached
zero, then the venting of gas continues and the process returns to step 28, i.e.,
the pressure and temperature of the gas in the fuel tank are measured again after
the passage of a second time interval as previously described.
[0038] FIG. 6 is a block diagram identifying components of a system for measuring a level
of liquid fuel in a fuel tank in accordance with the embodiments described above.
All calculations are performed by a processing unit 44 which receives measurement
data from the pressure gauge 12, the temperature gauge 14, a fuel flow meter 48 and
a gas flow meter 50. The processing unit 44 is programmed to execute algorithms for
quantifying the amount of fuel in a fuel tank. The processing unit 44 outputs the
fuel quantity data to a fuel gauge 46, which displays symbology representing the quantity
of fuel. The fuel gauge 46 may take the form of a display device having a display
processor programmed to display the measurement results (e.g., the fuel level) graphically
and/or alphanumerically on a display screen. The readings provided by the processing
unit 44 to the fuel gauge 46 may be integrated or averaged before presentation and
may be provided in real time substantially continuously or at different time intervals.
[0039] The processing unit 44 may be a dedicated microprocessor or a general-purpose computer.
In accordance with one embodiment, the algorithms executed by the processing unit
44 include: (1) an algorithm for computing a change in volume ΔV of fuel in the fuel
tank during a time interval by integrating a rate of flow of fuel out of the fuel
tank (provided by a fuel flow meter 48) during the time interval; (2) an algorithm
for computing a change in mass ΔM of gas in the fuel tank during the time interval
by integrating a rate of flow of gas into or out of the fuel tank (provided by a gas
flow meter 50) during the time interval; (3) an algorithm for computing the ullage
gas mass M and ullage gas volume V at the start of the time interval based on ΔV,
ΔM, pressure measurements taken at the start and end of the time interval by the pressure
gauge 12, and temperature measurements taken at the start and end of the time interval
by the temperature gauge 14; and (4) an algorithm for calculating the fuel volume
at the end of the time interval based on (V + ΔV) and the fixed volume of the fuel
tank. The fuel weight can be computed as the product of the fuel volume and the fuel
density.
[0040] One advantage of the system described above is that it measures the ullage, not the
fuel level (i.e., fuel surface location). Neither the attitude of the aircraft nor
the flight condition affect the reading or accuracy of the measurement.
[0041] Most aircraft have multiple fuel tanks. Similar to the current fuel system practice,
each fuel tank can be provided with its own individual fuel quantity gauging system.
For fuel tanks with multiple partitions, more than one temperature probe may be required.
[0042] Ullage gases are not limited to OBIGGS nitrogen-enriched air or engine bleed air.
Other gases or inert gas such as carbon dioxide can be used. For non-aircraft and
stationary tank applications, pressurized air or bottled gas can be used.
[0043] Furthermore, ullage gas is not limited to ideal gas. Any gas can be used. For example,
when a Van Der Waals gas is used, the Van Der Waals equation and the corresponding
Van Der Waals constants for the gas can be used.
[0044] Mixing two different gases, i.e., the gas added is different from what is in the
fuel tank, works but it may make the final calculation more tedious.
[0045] All aircraft systems are required to have backup system for redundancy. This fuel
gauging system is lightweight and inexpensive. Redundancy can be achieved by duplicating
the entire system in the fuel tanks as long as they are independent of each other.
[0046] While methods for measuring the quantity of liquid fuel in a fuel tank on an aircraft
have been described with reference to various embodiments, it will be understood by
those skilled in the art that various changes may be made and equivalents may be substituted
for elements thereof without departing from the teachings herein. In addition, many
modifications may be made to adapt the concepts and reductions to practice disclosed
herein to a particular situation. Accordingly, it is intended that the subject matter
covered by the claims not be limited to the disclosed embodiments.
[0047] In addition, the method claims set forth hereinafter should not be construed to require
that the steps recited therein be performed in alphabetical order (any alphabetical
ordering in the claims is used solely for the purpose of referencing previously recited
steps) or in the order in which they are recited. Nor should they be construed to
exclude any portions of two or more steps being performed concurrently or alternatingly.
1. An aircraft system for measuring a quantity of liquid fuel (10) in a fuel tank (4)
on an aircraft, comprising:
a first meter (50) that measures a rate of flow of gas through a gas line that is
in fluid communication with the fuel tank (4);
a second meter (48) that measures a rate of flow of liquid fuel (10) out of the fuel
tank (4) via a fuel line;
a first gauge (14) that measures an ullage temperature in an ullage (8) of the fuel
tank (4);
a second gauge (12) that measures an ullage pressure in the ullage (8) of the fuel
tank (4); and
a processing unit (44) programmed to calculate a quantity of liquid fuel (10) in the
fuel tank (4) based on measurement data from said first and second meters (50/48)
and from said first and second gauges (14/12).
2. The system as recited in claim 1, further comprising a fuel gauge (46) connected to
receive and display symbology representing said quantity of liquid fuel (10).
3. The system as recited in any of claims 1 to 2, wherein said processing unit (44) is
programmed to calculate a quantity of liquid fuel (10) in the fuel tank (4) at a second
time subsequent to a first time based in part on respective ullage temperature and
pressure measurements taken by said first and second gauges (14/12) at said first
and second times.
4. The system as recited in any of claims 1 to 3, wherein said processing unit (44) is
programmed to:
calculate a change in mass of gas in the fuel tank (4) by integrating an output of
said first meter (50) over a time interval from a first time to a second time;
calculate a change in volume of liquid fuel (10) in the fuel tank (4) by integrating
an output of said second meter 48 over said time interval; and
calculate a quantity of liquid fuel (10) in the fuel tank (4) at said second time
based on the calculated changes in mass of gas and fuel (10) in the fuel tank (4)
during said time interval and respective ullage temperature and pressure measurements
taken by said first and second gauges (14/12) at said first and second times.
5. The system as recited in any of claims 1 to 4, wherein said processing unit (44) is
further programmed to cause said gas line to be closed in response to an ullage pressure
that equals a preset maximum pressure.
6. The system as recited in any of claims 1 to 5, wherein said gas line is connected
to an on-board inert gas generation system.
7. The system as recited in any of claims 1 to 6, wherein said gas line receives engine
bleed air.
8. The system as recited in any of claims 1 to 7, wherein said gas line is a vent (16).
9. A method for measuring a quantity of liquid fuel (10) in a fuel tank (4) on an aircraft
while the liquid fuel (10) is flowing out of the fuel tank (4), comprising:
(a) changing a volume of gas in an ullage (8) above the liquid fuel (10) in the fuel
tank (4) during a time interval that starts at a first time and ends at a second time;
(b) measuring a rate of change of the volume of gas in the fuel tank (4) during said
time interval;
(c) measuring a rate of flow of the liquid fuel (10) out of the fuel tank (4) during
said time interval;
(d) measuring a first pressure of gas in the fuel tank (4) at said first time;
(e) measuring a first temperature of gas in the fuel tank (4) at said first time;
(f) measuring a second pressure of gas in the fuel tank (4) at said second time;
(g) measuring a second temperature of gas in the fuel tank (4) at said second time;
and
(h) calculating a quantity of the liquid fuel (10) in the fuel tank (4) based on measurement
data acquired in steps (b) through (g),
wherein step (h) is performed by a processing unit (44), preferably symbology representing
the quantity of the liquid fuel (10) is displayed.
10. The method as recited in claim 9, further comprising closing a vent (16) in fluid
communication with the ullage (8) prior to step (a), wherein step (a) comprises injecting
gas into the fuel tank (4) via a gas line during said time interval while the vent
(16) is closed, and step (b) comprises measuring a rate of flow of gas into the fuel
tank (4) via the gas line.
11. The method as recited in any of claims 9 to 10, further comprising closing the gas
line in response to an ullage pressure that equals a preset maximum pressure.
12. The method as recited in any of claims 9 to 11, further comprising opening a vent
(16) in fluid communication with the ullage (8) prior to step (a), wherein step (a)
comprises venting gas out of the ullage (8) via the open vent (16) during said time
interval, and step (b) comprises measuring a rate of flow of gas out of the ullage
(8) via the open vent (16), and closing said vent (16) in response to an ullage pressure
that equals zero.
13. The method as recited in any of claims 9 to 12, wherein step (h) comprises:
calculating a change in mass of gas in the fuel tank (4) during said time interval;
calculating a change in volume of fuel in the fuel tank (4) during said time interval;
and
calculating the quantity of the liquid fuel (10) in the fuel tank (4) at said second
time based on the calculated changes in mass of gas and the liquid fuel (10) in the
fuel tank (4) during said time interval and said first and second temperatures and
said first and second pressures.
14. The method as recited in any of claims 9 to 13, wherein said injected gas is nitrogen-enriched
air or engine bleed air.
1. Flugzeugsystem zum Messen einer Menge an flüssigem Kraftstoff (10) in einem Kraftstofftank
(4) in einem Flugzeug, umfassend:
einen ersten Zähler (50), der eine Durchflussrate von Gas durch eine Gasleitung misst,
die in Fluidverbindung mit dem Kraftstofftank (4) steht;
einen zweiten Zähler (48), der eine Durchflussrate von flüssigem Kraftstoff (10) aus
dem Kraftstofftank (4) durch eine Kraftstoffleitung misst;
ein erstes Messgerät (14), das eine Raum-Temperatur in einem füllungsfreien Raum (8)
des Kraftstofftanks (4) misst;
ein zweites Messgerät (12), das einen Raum-Druck in dem füllungsfreien Raum (8) des
Kraftstofftanks (4) misst; und
eine Verarbeitungseinheit (44), die programmiert ist, um eine Menge an flüssigem Kraftstoff
(10) im Kraftstofftank (4) basierend auf Messdaten von dem ersten und dem zweiten
Zähler (50/48) und von dem ersten und dem zweiten Messgerät (14/12) zu berechnen.
2. System nach Anspruch 1, ferner umfassend ein Kraftstoff-Messgerät (46), das verbunden
ist, um Symbole zu empfangen und anzuzeigen, die die Menge an flüssigem Kraftstoff
(10) darstellen.
3. System nach einem der Ansprüche 1 bis 2, wobei die Verarbeitungseinheit (44) programmiert
ist, um eine Menge an flüssigem Kraftstoff (10) im Kraftstofftank (4) zu einem zweiten
Zeitpunkt nach einem ersten Zeitpunkt zu berechnen, der teilweise auf entsprechenden
Raum-Temperatur- und Druckmessungen basiert, die von den ersten und zweiten Messgeräten
(14/12) zu dem ersten und zweiten Zeitpunkt durchgeführt werden.
4. System nach einem der Ansprüche 1 bis 3, wobei die Verarbeitungseinheit (44) programmiert
ist, zum
Berechnen einer Änderung der Gas-Masse im Kraftstofftank (4) durch Integrieren einer
Ausgabe des ersten Zählers (50) über ein Zeitintervall von einem ersten Zeitpunkt
zu einem zweiten Zeitpunkt;
Berechnen einer Volumenänderung des flüssigen Kraftstoffs (10) im Kraftstofftank (4)
durch Integrieren eines Ausgangssignals des zweiten Zählers 48 über das Zeitintervall;
und
Berechnen einer Menge an flüssigem Kraftstoff (10) im Kraftstofftank (4) zu dem zweiten
Zeitpunkt basierend auf den berechneten Massenänderungen von Gas und Kraftstoff (10)
im Kraftstofftank (4) während des Zeitintervalls und entsprechenden Raum-Temperatur-
und Raum-Druckmessungen, die von der ersten und zweiten Messgeräten (14/12) zu dem
ersten und zweiten Zeitpunkt durchgeführt werden.
5. System nach einem der Ansprüche 1 bis 4, wobei die Verarbeitungseinheit (44) ferner
so programmiert ist, dass die Gasleitung als Reaktion auf einen Raum-Druck, der einem
voreingestellten Maximaldruck entspricht, geschlossen wird.
6. System nach einem der Ansprüche 1 bis 5, wobei die Gasleitung mit einem fahrzeugseitigen
Schutzgaserzeugungssystem verbunden ist.
7. System nach einem der Ansprüche 1 bis 6, wobei die Gasleitung Triebwerks-Zapfluft
empfängt.
8. System nach einem der Ansprüche 1 bis 7, wobei die Gasleitung eine Entlüftung (16)
ist.
9. Verfahren zum Messen einer Menge an flüssigem Kraftstoff (10) in einem Kraftstofftank
(4) in einem Flugzeug, während der flüssige Kraftstoff (10) aus dem Kraftstofftank
(4) austritt, umfassend:
(a) Ändern eines Gasvolumens in einem füllungsfreien Raum (8) über dem flüssigen Kraftstoff
(10) im Kraftstofftank (4) während eines Zeitintervalls, das zu einem ersten Zeitpunkt
beginnt und zu einem zweiten Zeitpunkt endet;
(b) Messen einer Änderungsrate des Gasvolumens im Kraftstofftank (4) während des Zeitintervalls;
(c) Messen einer Strömungsgeschwindigkeit des flüssigen Kraftstoffs (10) aus dem Kraftstofftank
(4) während des Zeitintervalls;
(d) Messen eines ersten Gasdrucks im Kraftstofftank (4) zu dem ersten Zeitpunkt;
(e) Messen einer ersten Gastemperatur im Kraftstofftank (4) zu dem ersten Zeitpunkt;
(f) Messen eines zweiten Gasdrucks im Kraftstofftank (4) zu dem zweiten Zeitpunkt;
(g) Messen einer zweiten Gastemperatur im Kraftstofftank (4) zu dem zweiten Zeitpunkt;
und
(h) Berechnen einer Menge des flüssigen Kraftstoffs (10) im Kraftstofftank (4) basierend
auf den in den Schritten (b) bis (g) erfassten Messdaten,
wobei Schritt (h) durch eine Verarbeitungseinheit (44) durchgeführt wird, vorzugsweise
wobei Symbole, die die Menge des flüssigen Kraftstoffs (10) darstellen, angezeigt
werden.
10. Verfahren nach Anspruch 9, ferner umfassend Schließen einer Entlüftung (16) in Fluidverbindung
mit dem füllungsfreien Raum (8) vor Schritt (a), wobei Schritt (a) das Einspritzen
von Gas in den Kraftstofftank (4) über eine Gasleitung während des Zeitintervalls
umfasst, während die Entlüftung (16) geschlossen ist, und Schritt (b) das Messen einer
Gasflussrate in den Kraftstofftank (4) über die Gasleitung umfasst.
11. Verfahren nach einem der Ansprüche 9 bis 10, ferner umfassend Schließen der Gasleitung
als Reaktion auf einen Flüssigkeitsdruck, der einem voreingestellten maximalen Druck
entspricht.
12. Verfahren nach einem der Ansprüche 9 bis 11, ferner umfassend Öffnen einer Entlüftung
(16) in Fluidverbindung mit dem füllungsfreien Raum (8) vor Schritt (a), wobei Schritt
(a) das Entlüften von Gas aus dem füllungsfreien Raum (8) über die offene Entlüftung
(16) während des Zeitintervalls umfasst, und Schritt (b) das Messen einer Gasflussrate
aus dem füllungsfreien Raum (8) über die offene Entlüftung (16) und das Schließen
der Entlüftung (16) als Reaktion auf einen Druck im füllungsfreien Raum, der gleich
Null ist, umfasst.
13. Verfahren nach einem der Ansprüche 9 bis 12, wobei Schritt (h) umfasst:
Berechnen einer Änderung der Gas-Masse im Kraftstofftank (4) während des Zeitintervalls;
Berechnen einer Änderung des Kraftstoffvolumens im Kraftstofftank (4) während des
Zeitintervalls; und
Berechnen der Menge des flüssigen Kraftstoffs (10) im Kraftstofftank (4) zu dem zweiten
Zeitpunkt basierend auf den berechneten Massenänderungen von Gas und dem flüssigen
Kraftstoff (10) im Kraftstofftank (4) während des Zeitintervalls und der ersten und
zweiten Temperaturen und des ersten und zweiten Drucks.
14. Verfahren nach einem der Ansprüche 9 bis 13, wobei das eingespritzte Gas stickstoffangereicherte
Luft oder Treibwerks-Zapfluft ist.
1. Système d'aéronef pour mesurer une quantité de carburant liquide (10) dans un réservoir
de carburant (4) sur un aéronef, comprenant :
un premier appareil de mesure (50) qui mesure un débit de gaz à travers une conduite
de gaz qui est en communication fluidique avec le réservoir de carburant (4) ;
un deuxième appareil de mesure (48) qui mesure un débit de carburant liquide (10)
sortant du réservoir de carburant (4) via une conduite de carburant ;
une première jauge (14) qui mesure une température de volume mort dans un volume mort
(8) du réservoir de carburant (4) ;
une deuxième jauge (12) qui mesure une pression de volume mort dans le volume mort
(8) du réservoir de carburant (4) ; et
une unité centrale (44) programmée de façon à calculer une quantité de carburant liquide
(10) dans le réservoir de carburant (4) en se basant sur les données de mesure venant
desdits premier et deuxième appareils de mesure (50/48) et venant desdites première
et deuxième jauges (14/12).
2. Système selon la revendication 1, comprenant en outre une jauge de carburant (46)
reliée de façon à recevoir et à afficher une symbologie représentant ladite quantité
de carburant liquide (10).
3. Système selon l'une quelconque des revendications 1 et 2, dans lequel ladite unité
centrale (44) est programmée de façon à calculer une quantité de carburant liquide
(10) dans le réservoir de carburant (4) à un deuxième moment ultérieur à un premier
moment en se basant partiellement sur les mesures respectives de température et de
pression du volume mort prises par lesdites première et deuxième jauges (14/12) auxdits
premier et deuxième moments.
4. Système selon l'une quelconque des revendications 1 à 3, dans lequel ladite unité
centrale (44) est programmée de façon à :
calculer un changement de la masse de gaz dans le réservoir de carburant (4) en intégrant
une sortie dudit premier appareil de mesure (50) sur un intervalle de temps allant
du premier moment au deuxième moment ;
calculer un changement du volume de carburant liquide (10) dans le réservoir de carburant
(4) en intégrant une sortie dudit deuxième appareil de mesure (48) sur ledit intervalle
de temps ; et à
calculer une quantité de carburant liquide (10) dans le réservoir de carburant (4)
audit deuxième moment en se basant sur les changements calculés de masse de gaz et
de carburant (10) dans le réservoir de carburant (4) pendant ledit intervalle de temps
et sur les mesures respectives de température et de pression du volume mort prises
par lesdites première et deuxième jauges (14/12) auxdits premier et deuxième moments.
5. Système selon l'une quelconque des revendications 1 à 4, dans lequel ladite unité
centrale (44) est programmée en outre pour faire en sorte que la conduite de gaz soit
fermée en réponse à une pression du volume mort qui est égale à une pression maximum
prédéfinie.
6. Système selon l'une quelconque des revendications 1 à 5, dans lequel ladite conduite
de gaz est reliée à un système de production de gaz inerte à bord.
7. Système selon l'une quelconque des revendications 1 à 6, dans lequel ladite conduite
de gaz reçoit de l'air de prélèvement du moteur.
8. Système selon l'une quelconque des revendications 1 à 7, dans lequel ladite conduit
de gaz est une prise d'air (16).
9. Procédé pour mesurer une quantité de carburant liquide (10) dans un réservoir de carburant
(4) sur un aéronef tandis que le carburant liquide (10) s'écoule hors du réservoir
de carburant (4), comprenant :
(a) le changement d'un volume de gaz dans un volume mort (8) au-dessus du carburant
liquide (10) dans le réservoir de carburant (4) pendant un intervalle de temps qui
commence à un premier moment et qui finit à un deuxième moment ;
(b) la mesure d'une vitesse de changement du volume de gaz dans le réservoir de carburant
(4) pendant ledit intervalle de temps ;
(c) la mesure d'un débit du carburant liquide (10) sortant du réservoir de carburant
(4) pendant ledit intervalle de temps ;
(d) la mesure d'une première pression du gaz dans le réservoir de carburant (4) audit
premier moment ;
(e) la mesure d'une première température du gaz dans le réservoir de carburant (4)
audit premier moment ;
(f) la mesure d'une deuxième pression du gaz dans le réservoir de carburant (4) audit
deuxième moment ;
(g) la mesure d'une deuxième température du gaz dans le réservoir de carburant (4)
audit deuxième moment ; et
(h) le calcul d'une quantité du carburant liquide (10) dans le réservoir de carburant
(4) en se basant sur les données de mesure acquises lors des étapes (b) à (g),
l'étape (h) étant exécutée par une unité centrale (44), de préférence une symbologie
représentant la quantité du carburant liquide (10) étant affichée.
10. Procédé selon la revendication 9, comprenant en outre la fermeture d'une prise d'air
(16) en communication fluidique avec le volume mort (8) avant l'étape (a), l'étape
(a) comprenant l'injection de gaz dans le réservoir de carburant (4) via une conduite
de gaz pendant ledit intervalle de temps tandis que la prise d'air (16) est fermée,
et l'étape (b) comprenant la mesure d'un débit de gaz pénétrant dans le réservoir
de carburant (4) via la conduite de gaz.
11. Procédé selon l'une quelconque des revendications 9 et 10, comprenant en outre la
fermeture de la conduite de gaz en réponse à une pression de volume mort qui est égale
à une pression maximum prédéfinie.
12. Procédé selon l'une quelconque des revendications 9 à 11, comprenant en outre l'ouverture
d'une prise d'air (16) en communication fluidique avec le volume mort (8) avant l'étape
(a), l'étape (a) comprenant la mise à l'air libre du gaz hors du volume mort (8) via
la prise d'air ouverte (16) pendant ledit intervalle de temps, et l'étape (b) comprenant
la mesure d'un débit de gaz sortant du volume mort (8) via la prise d'air ouverte
(16) et la fermeture de ladite prise d'air (16) en réponse à une pression de volume
mort qui est égale à zéro.
13. Procédé selon l'une quelconque des revendications 9 à 12, dans lequel l'étape (h)
comprend :
le calcul d'un changement de la masse de gaz dans le réservoir de carburant (4) pendant
ledit intervalle de temps ;
le calcul d'un changement du volume du carburant dans le réservoir de carburant (4)
pendant ledit intervalle de temps ; et
le calcul de la quantité du carburant liquide (10) dans le réservoir de carburant
(4) audit deuxième moment en se basant sur les changements calculés de masse de gaz
et de carburant liquide (10) dans le réservoir de carburant (4) pendant ledit intervalle
de temps et sur lesdites première et deuxième températures et lesdites première et
deuxième pressions.
14. Procédé selon l'une quelconque des revendications 9 à 13, dans lequel ledit gaz injecté
est de l'air enrichi en azote ou de l'air de prélèvement du moteur.